This invention relates generally to actuators and corresponding methods and systems for controlling such actuators, and in particular, to actuators providing independent lift and timing control.
In general, various systems can be used to actively control engine valves through the use of variable lift and/or variable timing so as to achieve various improvements in engine performance, fuel economy, reduced emissions, and other like aspects. Depending on the means of the control or the actuator, they can be classified as mechanical, electrohydraulic, electro-mechanical, etc. Depending on the extent of the control, they can be classified as variable valve-lift and timing (VVLT), variable valve-timing (VVT), and variable valve-lift (VVL).
Both lift and timing of the engine valves can be controlled by some mechanical systems. The lift and timing controls are generally, however, not independent, and the systems typically have only one-degree of freedom. Such systems are therefore not VVLT per se and are often more appropriately designated as variable valve-actuation (VVA) systems. Electro-mechanical VVT systems generally replace the cam in the mechanical VVLT system with an electro-mechanical actuator. However, such systems do not provide for variable lift.
In contrast, an electrohydraulic VVLT system is controlled by electrohydraulic valves, and can generally achieve independent timing and lift controls so as to thereby provide greater control capability and power density. However, typical electrohydraulic VVLT systems are generally rather complex, can be expensive to manufacture, and typically are not as reliable or robust as mechanical systems due to their relative complexity.
A true VVLT system has two degrees of freedom and offers the maximum flexibility to engine control strategy development. Typically, such systems require, for each engine valve or each pair of engine valves, at least two high-performance electrohydraulic flow control valves and a fast responding position sensing and control system, which can result in high costs and complexity.
For these reasons, typical control systems are not able to control engine valve lift and timing independently with a simple and cost effective design for mass production. Moreover, for non-hydraulic systems, it can be difficult to provide lash adjustment, which is to perform a longitudinal mechanical adjustment so that an engine valve is properly seated.
Briefly stated, in one aspect of the invention, one preferred embodiment of an actuator comprises a cylinder, a first, second and third port, an actuation piston, a control piston and a control spring. The cylinder defines a longitudinal axis and comprises a first and second end. The first port communicates with the first end of the cylinder, the second port communicates with the second end of the cylinder, and the third port communicates with the cylinder between the first and second ends. The actuation piston is disposed in the cylinder and is moveable along the longitudinal axis in a first and second direction. The actuation piston comprises a first and second side. The control piston also is disposed in the cylinder and is moveable along the longitudinal axis in a first and second direction. The control piston comprises a first and second side, with the first side of the control piston facing the second side of the actuation piston. The control spring biases the control piston in at least one of the first and second directions.
In one preferred embodiment, a first chamber is formed between the first end of the cylinder and the first side of said actuation piston, a second chamber is formed between the second side of the control piston and the second end of the cylinder, a third chamber is formed between the second side of the actuation piston and the first side of the control piston. In alternative preferred embodiments, one of the second and third chambers forms an exhaust chamber, while the other of the second and third chambers forms a control chamber.
In one preferred embodiment, the first port is connected alternatively with a high pressure line and a low pressure exhaust line in a fluid supply assembly through an on/off valve when the valve is electrically energized and unenergized. The timing of the actuation is thus varied through the timing control of the on/off valve. One of the second and third ports, configured as a control port, is connected with a control pressure regulating assembly and thus under a control pressure. The other of the second and third ports, configured as an exhaust port, is connected with the exhaust line. In between the exhaust port and the exhaust chamber, there is a lift flow restrictor that exerts substantial resistance to flow through it. Because of the lift flow restrictor, pressure inside the exhaust chamber can be substantially different from that at the exhaust port under dynamic situations. As a result, the lift flow restrictor can make it difficult to move the control piston at a substantial speed. At its nominal position, the control piston is primarily balanced by the control pressure force and the control spring force. The nominal position of the control piston is thus regulated by the control pressure, and the position is not much or slowly changed under dynamic situations because of the lift flow restrictor.
In one preferred embodiment, the fluid actuator is applied to the control of the intake and exhaust valves of an internal combustion engine, wherein a piston rod, which is connected to the actuation piston, is connected to an engine valve stem. The engine valve is primarily pushed up or seated on a valve seat by a return spring and driven down, or opened, by the actuator.
In other aspects of the invention, methods of controlling the actuator are also provided.
The present invention provides significant advantages over other actuators and valve control systems, and methods for controlling actuators and/or valve engines. The incorporations of a second (control) piston, a control spring, a lift flow restrictor, and a control pressure port in an otherwise conventional single-piston-rod fluid actuator, provides a simple but robust actuator in which timing and lift can be independently controlled. In particular, the nominal position of the control piston is determined primarily by the force balance between the control pressure and the control spring. The stroke or lift of the actuation piston is determined by the position of the control piston. Even when being pushed by the actuation piston, the control piston is able to stay, for a short but sufficient period of time, substantially at its nominal position.
In addition, although the actuation time for a typical engine valve is very fast and is in the range of a few milliseconds, that fast time response is not required to change the lift of the valve. Rather, the actuators of the present invention use a simple control piston/control spring mechanism to achieve the lift control. The control pressure for all actuators of the intake valves or exhaust valves or both of an entire internal combustion engine can be regulated by a single pressure regulator, the cost of which is thus spread over the entire engine. Only a simple switch valve per fluid actuator is needed to control the actuation. There is no need for sophisticated position sensing and control.
In addition, in conventional systems, in order to achieve a closed loop position feedback control during a short period of time, super fast hydraulic switch valves are needed. With the open loop approach of the present invention, the hydraulic switch valves are not required to have a super fast time response.
The present invention, together with further objects and advantages, will be best understood by reference to the following detailed description taken in conjunction with the accompanying drawings.